When Is Embryonic Arrest Broken in Turtle Eggs?

Effects of postovipositional hypoxia and hyperoxia on leatherback turtle reproductive success and hatchling performance

Leatherback egg clutches typically experience lower hatching success (~50%) than those of other sea turtle species (>70%). The majority of embryonic death (>50%) occurs at early stages of development, possibly because embryos fail to break preovipositional embryonic arrest after oviposition. The embryonic arrest is maintained by hypoxia in the oviduct and following oviposition increased availability of oxygen is the trigger that breaks arrest in all turtle species studied to date. We conducted an ex situ incubator experiment and an in situ hatchery experiment to examine the influence of oxygen availability on hatching success and hatchling traits in leatherbacks. After oviposition, eggs (n = 1005) were incubated in either normoxia (21% O2 ), hyperoxia (32%-42% O2 ) for 5 days, or hypoxia (1% O2 ) for 3 or 5 days. As with other turtles, hypoxic incubation maintained embryos in arrest, equivalent to the time spent in hypoxia. However, extending arrest for 5 days resulted in greater early-stage death and a significant decrease in hatching success (4% 5-day hypoxia vs. 72% normoxia). Eggs placed in incubators had greater hatching success than those placed into hatchery nests (67% vs. 47%, respectively). We found no impact of hyperoxia on the stage of embryonic death, hatching success, hatchling phenotype, exercise performance, or early dispersal. Our findings indicate that delayed nesting and the subsequent extension of embryonic arrest may negatively impact embryonic development and therefore the reproductive success of leatherbacks. They also indicate that incubation under hyperoxic conditions is unlikely to be a useful method to improve hatching success in this species.

Increasing hypoxia progressively slows early embryonic development in an oviparous reptile, the green turtle, Chelonia mydas

Green turtle (Chelonia mydas) embryos are in an arrested state of development when the eggs are laid, but in the presence of oxygen, arrest is broken and development resumes within 12-16 h. However, the precise oxygen level at which embryos break arrest and continue development is not known. To better understand the impact of oxygen concentration on breaking of arrest and early embryonic development, we incubated freshly laid eggs of the green sea turtle for three days at each of six different oxygen concentrations (less than or equal to 1%, 3%, 5%, 7%, 9% and 21%) and monitored the appearance and growth of white spots on the shell, indicative of embryonic development. As reported previously, white spots did not develop on eggs incubated in anoxia (less than or equal to 1% oxygen). For all other treatments, mean time to white spot detection and white spot growth rate varied inversely with oxygen concentration. In nearly all cases the difference between eggs at different oxygen levels was statistically significant (p ≤ 0.05). This suggests that sea turtle embryonic development may respond to oxygen in a dose-dependent manner. Our results indicate that the development of green turtle embryos may be slowed if they are exposed to the most hypoxic conditions reported in mature natural nests.

Transcriptomic analysis of pre-ovipositional embryonic arrest in a non-squamate reptile (Chelonia mydas)

After gastrulation, oviductal hypoxia maintains turtle embryos in an arrested state prior to oviposition. Subsequent exposure to atmospheric oxygen upon oviposition initiates recommencement of embryonic development. Arrest can be artificially extended for several days after oviposition by incubation of the egg under hypoxic conditions, with development recommencing in an apparently normal fashion after subsequent exposure to normoxia. To examine the transcriptomic events associated with embryonic arrest in green sea turtles (Chelonia mydas), RNA‐sequencing analysis was performed on embryos from freshly laid eggs and eggs incubated in either normoxia (oxygen tension ~159 mmHg) or hypoxia (<8 mmHg) for 36 h after oviposition (n = 5 per group). The patterns of gene expression differed markedly among the three experimental groups. Normal embryonic development in normoxia was associated with upregulation of genes involved in DNA replication, the cell cycle, and mitosis, but these genes were commonly downregulated after incubation in hypoxia. Many target genes of hypoxia inducible factors, including the gene encoding insulin‐like growth factor binding protein 1 (igfbp1), were downregulated by normoxic incubation but upregulated by incubation in hypoxia. Notably, some of the transcriptomic effects of hypoxia in green turtle embryos resembled those reported to be associated with hypoxia‐induced embryonic arrest in diverse taxa, including the nematode Caenorhabditis elegans and zebrafish (Danio rerio). Hypoxia‐induced preovipositional embryonic arrest appears to be a unique adaptation of turtles. However, our findings accord with the proposition that the mechanisms underlying hypoxia‐induced embryonic arrest per se are highly conserved across diverse taxa.

A review of the effects of incubation conditions on hatchling phenotypes in non-squamate reptiles

Developing embryos of oviparous reptiles show substantial plasticity in their responses to environmental conditions during incubation, which can include altered sex ratios, morphology, locomotor performance and hatching success. While recent research and reviews have focused on temperature during incubation, emerging evidence suggests other environmental variables are also important in determining hatchling phenotypes. Understanding how the external environment influences development is important for species management and requires identifying how environmental variables exert their effects individually, and how they interact to affect developing embryos. To address this knowledge gap, we review the literature on phenotypic responses in oviparous non-squamate (i.e., turtles, crocodilians and tuataras) reptile hatchlings to temperature, moisture, oxygen concentration and salinity. We examine how these variables influence one another and consider how changes in each variable alters incubation conditions and thus, hatchling phenotypes. We explore how incubation conditions drive variation in hatchling phenotypes and influence adult populations. Finally, we highlight knowledge gaps and suggest future research directions.

Ontogeny and ecological significance of metabolic rates in sea turtle hatchlings

Background

Sea turtle hatchlings must avoid numerous predators during dispersal from their nesting beaches to foraging grounds. Hatchlings minimise time spent in predator-dense neritic waters by swimming almost continuously for approximately the first 24 h post-emergence, termed the ‘frenzy’. Post-frenzy, hatchling activity gradually declines as they swim in less predator-dense pelagic waters. It is well documented that hatchlings exhibit elevated metabolic rates during the frenzy to power their almost continuous swimming, but studies on post-frenzy MRs are sparse.

Results

We measured the frenzy and post-frenzy oxygen consumption of hatchlings of five species of sea turtle at different activity levels and ages to compare the ontogeny of mass-specific hatchling metabolic rates. Maximal metabolic rates were always higher than resting metabolic rates, but metabolic rates during routine swimming resembled resting metabolic rates in leatherback turtle hatchlings during the frenzy and post-frenzy, and in loggerhead hatchlings during the post-frenzy. Crawling metabolic rates did not differ among species, but green turtles had the highest metabolic rates during frenzy and post-frenzy swimming.

Conclusions

Differences in metabolic rate reflect the varying dispersal stratagems of each species and have important implications for dispersal ability, yolk consumption and survival. Our results provide the foundations for links between the physiology and ecology of dispersal of sea turtles.

Effects of moisture during incubation on green sea turtle (Chelonia mydas) development, morphology and performance

While the effect of temperature on embryonic development in sea turtles has been well studied over recent years, our understanding of the effect of substrate moisture, another important environmental variable, is limited. High sand moisture decreases nest temperature through evaporative and direct cooling during rainfall, but its direct effect on hatchling development, morphology and performance is unclear. To address this knowledge gap, we incubated 40 green sea turtle Chelonia mydas clutches in a beach hatchery under either high (~8% v/v) or low (~5% v/v) sand moisture concentrations for the duration of embryonic development. In half of the clutches, temperature sensors were deployed to measure any effect of sand moisture on nest temperature. As hatchlings emerged, we measured body size and locomotory performance during the first 24 h, an important period of frenzied activity for sea turtles. We excavated clutches post-emergence to determine hatching success, emergence success and to determine the stage of embryonic death for unsuccessful eggs. High moisture concentrations increased incubation duration, decreased nest temperature and had marginal effects on hatchling morphology, but no effect on hatching success, stage of embryonic death, crawling speed or initial swimming performance. However, after 24 h of swimming, hatchlings from high-moisture clutches produced less mean swim thrust and spent less time powerstroking than hatchlings from low-moisture clutches, suggesting reduced swimming endurance and potentially impacting the ability of hatchlings to successfully disperse. The effect of moisture on nest temperature and hatchling endurance highlights the importance of considering rainfall patterns when predicting future impacts of climate change on sea turtle populations.

Metabolic Rates and Thermal Thresholds of Embryonic Flatback Turtles (Natator depressus) from the North West Shelf of Australia

AbstractNest microclimates influence embryonic development and survival in many lineages, including reptiles with temperature-dependent sex determination. These microclimates are dependent on physical drivers and biological processes, such as embryonic metabolism, that generate heat. The flatback turtle (Natator depressus) has among the largest hatchlings of the seven extant sea turtle species, making it an excellent candidate for quantifying the contribution of embryonic metabolism to the nest microclimate. Consequently, we measured embryonic metabolic rates, development rates, and the relationship between temperature and sex determination for a N. depressus population nesting at Cemetery Beach in Western Australia, a mainland beach characterized by high sand temperatures. Total oxygen consumed at 29.5°C during an average 52-d incubation period was 2,622 mL, total carbon dioxide produced was 1,886 mL, and estimated embryonic heat production reached 38 mW at 90% of development. Adjustment of metabolic rates to 32°C and 34°C increased peak heat production by 18% and 27%, respectively. The pivotal temperature (T_PIV) producing an equal sex ratio was 30.3°C, mixed sexes were produced between 29.3°C and 31.2°C, and only females were produced above 31.2°C. The T_PIV was similar (within 0.2°C) to that of an island rookery within the same genetic stock (North West Shelf), but the peak development rate (2.5% d^-1) was estimated to be achieved at a temperature ~2.5°C higher (34.7°C) than the island rookery. Our results add to a growing consensus that thermal thresholds vary among sea turtle populations, even within the same genetic stock. Furthermore, we show that metabolic heat will have an appreciable impact on the nest microclimate, which has implications for embryonic survival and fitness under a future climate with warmer sand temperatures.

Role of incubation environment in determining thermal tolerance of sea turtle hatchlings

Warming global temperatures are predicted to reduce population viability in many oviparous ectothermic taxa, with increased embryonic mortality likely to be a main cause. While research on embryonic upper thermal limits is extensive, sea turtle hatchling thermal tolerance has received less attention and our understanding of how incubation conditions influence hatchling thermal tolerance is limited. Here, we report green turtle Chelonia mydas hatchling hydration and thermal tolerance following incubation in dry and wet conditions. We used packed cell volume and total protein as indicators of hydration and measured the critical thermal maximum (CTmax) of hatchlings in air. Neither hydration nor thermal tolerance was directly influenced by moisture treatment. However, hatchlings from moister nests had longer incubation durations (wet: 60.11 d vs. dry: 54.86 d), and, using incubation duration as a proxy for incubation temperature, hatchlings from cooler nests had significantly lower CTmax (wet: 39.84°C vs. dry: 40.51°C). Thus, despite not directly influencing thermal tolerance, moisture treatment influenced nest temperature indirectly; hatchlings that experienced warmer conditions in dry nests had a higher thermal tolerance than hatchlings from cooler and wetter nests. Ectothermic neonates may have greater plasticity in their thermal tolerance than previously thought, but their ability to adapt to increasing temperature is likely limited. Additionally, common management techniques to reduce nest temperatures, such as watering and shading nests, may only reduce embryonic mortality at the cost of decreased hatchling thermal tolerance and increased hatchling mortality during emergence. Nesting-site management interventions designed to reduce embryonic mortality will need to consider mitigation of the possible effects of those interventions on hatchling mortality.

Flooding-induced mortality of loggerhead sea turtle eggs

Abstract ContextMarine turtle eggs incubate in dynamic beaches, where they are vulnerable to both saltwater and freshwater flooding. Understanding the capacity for marine turtle eggs to tolerate flooding will aid management efforts to predict and mitigate the impacts of climate change, including sea-level rise and increases in coastal flooding. AimsEvaluate the interactive effects of flooding duration and incubation stage on the hatching success of loggerhead turtle (Caretta caretta) eggs. MethodsGroups of 20 eggs from multiple clutches were incubated in plastic containers in a beach hatchery. Eggs at six stages of incubation (0, 1, 2, 4, 6 and 7 weeks post-oviposition) were excavated from the hatchery and exposed to saltwater or freshwater flooding for seven durations of time (0, 1, 2, 3, 6, 24 or 48h). Containers of eggs were either submerged in a bucket of water or left outside of the bucket (control; no flooding) for their designated duration, allowed to drain, then reburied in the hatchery. Following hatchling emergence, the hatching success of each group of eggs was evaluated. Key resultsFreshly laid eggs and eggs on the verge of hatching exposed to any flooding and all eggs exposed to extended periods of flooding (24 and 48h) suffered complete mortality. Eggs at 20–80% development exposed to short periods of flooding (1–6h) maintained high hatching success that was statistically equivalent to control eggs, while eggs at &lt;20% and &gt;80% development exhibited significant decreases in hatching success. ConclusionsMarine turtle eggs in the middle of incubation can tolerate saltwater and freshwater flooding for up to 6h. Outside of this period or when flooding is longer, disruption of gas concentrations and osmotic gradients in the egg chamber can lead to embryonic mortality. These findings have reinforced concerns regarding the capacity for marine turtle populations to continue to function as rising sea levels and increases in coastal flooding alter the hydrology of nesting beaches. ImplicationsAs current and predicted climate change threatens the suitability of the incubation environment used by marine turtles, corrective actions to maximise hatching success need to be taken before the eggs are flooded.

The effect of respiratory gases and incubation temperature on early stage embryonic development in sea turtles

Sea turtle embryos at high-density nesting beaches experience relative high rates of early stage embryo death. One hypothesis to explain this high mortality rate is that there is an increased probability that newly constructed nests are located close to maturing clutches whose metabolising embryos cause low oxygen levels, high carbon dioxide levels, and high temperatures. Although these altered environmental conditions are well tolerated by mature embryos, early stage embryos, i.e. embryos in eggs that have only been incubating for less than a week, may not be as tolerant leading to an increase in their mortality. To test this hypothesis, we incubated newly laid sea turtle eggs over a range of temperatures in different combinations of oxygen and carbon dioxide concentrations and assessed embryo development and death rates. We found that gas mixtures of decreased oxygen and increased carbon dioxide, similar to those found in natural sea turtle nests containing mature embryos, slowed embryonic development but did not influence the mortality rate of early stage embryos. We found incubation temperature had no effect on early embryo mortality but growth rate at 27°C and 34°C was slower than at 30°C and 33°C. Our findings indicate that low oxygen and high carbon dioxide partial pressures are not the cause of the high early stage embryo mortality observed at high-density sea turtle nesting beaches, but there is evidence suggesting high incubation temperatures, particularly above 34°C are harmful. Any management strategies that can increase the spacing between nests or other strategies such as shading or irrigation that reduce sand temperature are likely to increase hatching success at high-density nesting beaches.

A method for the collection of early-stage sea turtle embryos

Early-stage turtle embryos, immediately after oviposition, are very small (<5 mm diameter), hindering research on the initial period of embryonic development. For example, assessing whether turtle eggs had been fertilized and contained a viable embryo at oviposition, especially under field conditions, is complicated by the microscopic size of embryos that may have died at an early stage of development. Further, little is known about the molecular pathways that promote and regulate early developmental processes in turtles, such as pre-ovipositional embryonic arrest. To enable further investigation of the processes critical to early embryonic development in turtle species, a reliable method is required for extraction of early-stage embryos from the egg. Therefore, our aim was to develop a novel and reproducible method for extracting early-stage sea turtle embryos. Herein, we describe the technique for extracting Chelonia mydas embryos before and after white spot formation. Once the embryos were collected, the total RNA of 10 embryos was extracted to validate the method. The total RNA concentration was above 5 ng µl-1 and the RNA integrity number varied between 7.0 and 10.0, which is considered acceptable for further RNA-sequencing analyses. This extraction technique could be employed when investigating fertilization rates of turtle nests and for further investigation of the molecular biology of embryonic development in turtles. Furthermore, the technique should be adaptable to other turtle species or any oviparous species with similar eggs.

Assessing the evidence of ‘infertile’ sea turtle eggs

There is increasing concern about feminization of sea turtle populations resulting from female-biased production of hatchlings due to climate change and selective loss of males from other anthropogenic drivers. Extreme female-biased breeding populations would reduce the likelihood of successful mating and potentially result in high rates of infertile eggs. Infertile eggs are those in which none of the events between sperm penetration of the ovum and syngamy have occurred. Distinguishing between fertile and infertile eggs is challenging, especially in field conditions, and researchers often have relied on physical evidence gathered from unhatched eggs at the end of the incubation period, which likely have experienced tissue decomposition. We argue that infertility in sea turtle eggs can be demonstrated only by the absence of holes caused by sperm penetration of the inner perivitelline membrane; sperm bound between the inner and outer perivitelline membranes; nuclei in the blastodisc; embryonic tissue or membranes in egg contents; and/or the characteristic white spot on the egg exterior. Unhatched eggs can be examined at the end of the incubation period, but we recommend that studies specifically investigating infertility examine at least 20 oviposited eggs each from clutches laid by at least 20 different turtles at the peak of the nesting season.

Synchronised nesting aggregations are associated with enhanced capacity for extended embryonic arrest in olive ridley sea turtles

Sea turtle species in the genus Lepidochelys exhibit an unusual behavioural polymorphism, nesting in both aggregations and solitarily. Aggregated nesting events, termed ‘arribadas’, involve hundreds of thousands of females congregating at a single nesting beach over a few days to oviposit their eggs. Aggregate and solitary nesting behaviours are associated with distinct inter-nesting intervals, three and four weeks for non-arribada and arribada nesters respectively. Consequently, embryos are maintained in pre-ovipositional embryonic arrest in the hypoxic oviduct for different lengths of time depending on the mother’s reproductive behaviour. However, sea turtle embryos are limited in their capacity to remain in arrest and will subsequently die if held in hypoxia too long. Here, we tested whether embryos oviposited during arribada or non-arribada nesting differ in their capacity to be maintained in pre-ovipositional arrest. Olive ridley turtle (Lepidochelys olivacea) eggs from eight clutches (four from each nesting tactic) were divided among seven treatments after oviposition; normoxia (control; 21% O₂), or hypoxia (1% O₂) for 3, 3.5, 4, 8, 15 or 30 days, before being returned to normoxia. Arribada eggs were capable of extending pre-ovipositional arrest for longer, with some eggs from the 8- and 15-day hypoxia treatment still hatching while no non-arribada eggs hatched after more than four days in hypoxia. This difference in embryonic capacity to survive extended periods of arrest may be an important mechanism facilitating arribada behaviour by allowing longer inter-nesting intervals. Our finding provides an intriguing insight into the physiological mechanisms that are integral to this unique mass-nesting behaviour.

Manipulation of Developmental Function in Turtles with Notes on Alligators

Reptiles have great taxonomic diversity that is reflected in their morphology, ecology, physiology, modes of reproduction, and development. Interest in comparative and evolutionary developmental biology makes protocols for the study of reptile embryos invaluable resources. The relatively large size, seasonal breeding, and long gestation times of turtles epitomize the challenges faced by the developmental biologist. We describe protocols for the preparation of turtle embryos for ex ovo culture, electroporation, in situ hybridization, and microcomputed tomography. Because these protocols have been adapted and optimized from methods used for frog, chick, and mouse embryos, it is likely that they could be used for other reptilian species. Notes are included for alligator embryos where appropriate.

A preliminary investigation into the early embryo death syndrome (EEDS) at the world's largest green turtle rookery

Raine Island hosts the largest nesting aggregation of green turtles in the world, but nest emergence success and hence recruitment of hatchlings off the beach appear to have significantly declined since the 1990s. Nests destroyed by subsequent nesting turtles, and nest failure due to flooding account for most of the nest failure, but many nests still have poor hatch success even when undisturbed and flood-free. In undisturbed, flood-free nests that experience high mortality, embryos typically die at a very early stage of development, a phenomenon we term early embryo death syndrome (EEDS). Previous research indicates that EEDS is correlated with the number of females nesting at Raine Island during a nesting season. Here, we monitor nest temperature and oxygen (PO₂) and carbon dioxide (PCO₂) partial pressures during the first week after nest construction to discover if they are associated with EEDS. Our investigation found that the proportion of early embryo death was greatest in two nests that experienced the highest nest temperature, lowest PO₂ and highest PCO₂ during the first week of incubation suggesting that these variables either by themselves or in combination may be the underlying cause of EEDS. These two nests were located adjacent to maturing nests, so the high temperature and more extreme PO₂s and PCO₂s are most likely to be caused by the combined metabolism of embryos in the mature nests. Although this conclusion is based on just two nests and needs to be substantiated in future studies, it would appear that the laying of new nests in the close location to mature nests could be a significant cause of hatch failure at high density nesting sea turtle rookeries around the world.

Ecological and evolutionary significance of a lack of capacity for extended developmental arrest in crocodilian eggs

Hypoxia within the oviducts maintains embryonic arrest in turtles at the pre-ovipositional stage, which expands the timeframe over which nesting can occur without compromising embryo survival. The arrest can be extended post-oviposition through incubation of eggs in hypoxia. We determined whether crocodilian embryos have this same capacity. We also tested whether increased oxygen availability during incubation alters hatching success. We incubated freshly laid saltwater crocodile (Crocodylus porosus) eggs (N = 83) at 32°C in one of five treatments; control (normoxia; 21% O₂), 3-day and 6-day hypoxia (1% O₂), or 3-day and 6-day hyperoxia (42% O₂). Incubation (approx. 82 days) was then completed in normoxia. There was a significant effect of treatment on survival of embryos through to hatching (p < 0.001). The hypoxic treatments resulted in almost no hatching (6.7% and 0% survival for the 3- and 6-day treatments, respectively), while the hyperoxic and control treatments resulted in normal to high hatching success (86.6%, 100% and 64.2% for the control, 3- and 6-day hyperoxic treatments, respectively). Unlike turtles, hypoxic incubation of crocodile eggs failed to delay development. Our results provide the first experimental evidence that, unlike turtles, crocodiles do not exhibit embryonic arrest when incubated under hypoxic conditions immediately following oviposition. An absence of embryonic arrest is of ecological and evolutionary significance, as it implies that crocodilians lack an ability to avoid adverse environmental conditions through delayed nesting and that, unlike turtles, embryonic arrest may not be a potential explanation for the lack of viviparity in the order Crocodylia.